This invention relates to electrography, and in particular, to the generation of halftone images with reduced image artifacts and increased levels of gray by the use of a rotating magnetic brush with a hard magnetic carrier, in conjunction with a digital, multi-bit printhead and halftone rendering system capable of printing variable dot sizes.
Electrographic print engines are used in printers and copiers to provide one or more copies of documents. Analog print engines rely upon a light lens to focus an image onto a charged image carrying member. Light strikes the charged image carrying member, discharges it and leaves a latent image on the member. Such print engines produce acceptable continuous tone images when the latent image on the image member is developed with developer comprised of a toner and a hard magnetic carrier. See for example U.S. Pat. Nos. 4,473,029; 4,531,832; 4,546,060; and 5,376,492, whose disclosures are incorporated by reference. Such copiers can reproduce images of photographs that are acceptable because they provide multiple levels of gray.
With the advent of digital technology, many images are captured with charge coupled arrays or other digital apparatus that converts the image into a set of pixels. In pure binary machines, the pixel is either on (black) or off (white). Such techniques are well suited to reproducing text because the sizes of the individual pixels that make up text symbols are much smaller than the symbols and the symbols are best seen with high contrast edges. Thus, the human eye sees the text as a continuous image even though it is a collection of closely spaced dots.
However, binary electrographic print engines do not provide acceptable levels of gray for other images, such as photographs. Those skilled in the art have used halftone dots to emulate gray scale for reproducing images with continuous tones. Newspapers and magazines are common examples of halftone printing. The reader does not see the halftone dots because they may be as small as xc2xd,500th to ⅕,000th of an inch. Such small sizes are possible with ink and with newsprint and magazine media. However, such small sizes are virtually impossible with electrographic toner. Indeed, the toner particles themselves are larger than the size of halftone dots used by newspapers and magazines.
Conventional binary electrographic halftone print engines try to make the dots as small as possible. Conventional toner stations provide binary dots that are too large for acceptable halftone imaging. Hard dots, ideally having sharp edges, are also deficient when made with conventional binary arrays or rendering techniques using developer comprised of a toner and a hard magnetic carrier. The hard dots break up and do not provide the desired sharp edges. Accordingly, there is a need for a new electrographic print engine that provides better halftone imaging. Conventional binary electrographic print engines do not meet this need.
In the area of digital printing, all colors including black or gray are represented on paper as one or more gray levels where gray refers to a color density between no color and saturation. There are a number of algorithms for rendering halftone images. Digital printers commonly make a mark, usually in the form of a dot pixel, of a given, uniform size and at a specified resolution in marks per unit length, typically dots per inch (dpi), on paper. A digital printer emulates color intensity by placing marks, or dots, on the paper in a geometrical pattern. The effect is such that a group of dots and dot-less blank spots, when seen by the eye, gives a rendition of an intermediate color tone or density between the color of the initial paper stock, usually white, and total ink coverage, or a solid density halftone dot. It is conventional to arrange the dots in rows, where the distance between rows is known as line spacing, and determines the number of lines per inch (lpi). In the ensuing paragraphs, discussions will be made in terms of white paper stock; it is understood that white paper stock is used as an illustration and not as a limitation of any invention.
Continuous tone images contain an apparent continuum of gray levels. Some scenes, when viewed by humans, may require more than 256 discrete gray levels for each color to give the appearance of a continuum of gray levels from one shade to another.
As an approximation to continuous tone images, conventional digital print engines create pictorial or graphical images via halftone technology. Halftone pictorial or graphical images lower the high contrast between the paper stock and the toned electrographic image and thereby create a more visually pleasing image. Such halftone methods use a basic picture element (also known as a cell) on the recording or display surface. The cell consists of a jxc3x97k matrix of sub-elements (pixels or pels) where j and k are positive integers. A halftone image is reproduced by printing the respective sub-elements or leaving them blank. That is, by suitably distributing the printed marks in each cell. Such halftoning technology uses various rendering algorithms, such as those disclosed in U.S. Pat. Nos. 5,198,910, 5,258,849, and 5,260,807, the teachings of which are incorporated herein by reference in their entirety, to form, arrange and/or otherwise orient the marks so as to modulate the contrast between the dots and paper stock background to render the image more visually pleasing.
Halftone image processing algorithms are evaluated, in part, by their capability of delivering a complete gray scale at normal viewing distances. The capability of a particular process to reproduce high frequency renditions (fine detail) with high contrast modulation makes that procedure superior to one which reproduces the fine detail with lesser or no output contrast.
Another figure of merit of image processing algorithms is the ability to suppress visual details in the output image that are not part of the original image, but are the result of the image processing algorithm. Such details are called artifacts, and include false contours and false textures. False contours are the result of gray scale quantization steps which are sufficiently large to create a visible contour when the input image is truly a smooth, gradual variation from one gray level to another. False textures, and textures that are visual and change with rendered density, are artificial changes in the image texture which occur when input gray levels vary slowly and smoothly but the output generates an artificial boundary between the textural patterns for one gray level and the textural patterns for the next gray level. Commonly used processing algorithms include fixed level thresholding, adaptive thresholding, orthographic tone scale fonts, and electronic screening.
In creating halftone images, two factors are of prime consideration: the line screen frequency and the number of addressable picture elements, i.e., pixels. Once the line screen frequency is determined, the number of addressable pixels determines the number of definable, i.e., theoretical, gray levels. The definable gray levels for a binary system can be calculated by the following formula:
Number of gray levels=(dpi/lpi)2+1
The screen frequency (lpi) tends to be set high so that the size of the dots are small and not visually detectable at normal viewing distances. An obvious problem arises when the resolution of the dot matrix on the paper is not very high, for example, 100 dpi or less. In such cases the geometrical patterns for the cell become visible to the eye. In that case the viewer is distracted from the image by artifacts of geometrical patterns themselves and perceives the impression of an image of poor quality. The obvious solution to this problem is to work at very high resolutions, for example, 300 dpi or greater, so that those artifacts are less perceived and their negative effects become less glaring. However, in view of the above formula, having a high screen frequency means there is a tradeoff with respect to the number of pixels available to create gray levels. Therefore, it would be desirable to maximize the number of defined gray levels while at the same time keeping the dots as small as possible.
Although a given number of gray levels can be theoretically set by selection of the dpi and lpi parameters, the number of gray levels attained in actual practice is limited by the shortcomings of known electrographic methods. For example, the number of gray levels discernable to the human eye are limited by a lack of sharpness surrounding the edges of the electrographic image, the presence of small xe2x80x9csatellitexe2x80x9d particles around the edges of the image or in the general background areas, and also by the inability to properly tone small, individual dots. The sharpness of continuous tone images such as those that are produced by flash or scanning light exposure systems are compromised by the optics through which they are produced. The above shortcomings can adversely impact the ability of such electrographic methods to create a smooth gray scale in pictorial or graphic images.
In recent years, digital printing technology has evolved to provide printheads with the ability to substantially increase the number of pixels per halftone cell. See, for example, the multi-bit printheads disclosed in U.S. Pat. Nos. 5,300,960, 5,604,527, and 5,739,841, the teachings of which are incorporated herein by reference in their entirety. The number of gray levels that those printheads can theoretically print can be determined using the following formula:
Number of gray levels=1+(no. of pixels per cell)xc3x97(2nxe2x88x921)
where n is equal to the number of bits associated with the image writing device and the number of pixels per cell is determined by (dpi/lpi)2.
For example, where the image-writing device is a 4-bit digital printhead capable of 300 dpi, such as those illustrated in the foregoing patents, and n is equal to 4 in the above formula, the number of gray levels calculated for a 600 lpi screen frequency is equal to 121. The rendering programs previously mentioned herein employ mathematical algorithms which xe2x80x9cbuildxe2x80x9d the individual dots on the halftone image so as to create the gray levels, as is known in the art.
In electrography, an electrostatic charge image is formed on a dielectric surface, typically the surface of the photoconductive recording element. The image is developed by contacting it with a two-component developer comprising a mixture of pigmented resinous particles, known as toner, and magnetically attractable particles, known as carrier. The carrier particles serve as sites against which the non-magnetic toner particles can impinge and thereby acquire a triboelectric charge that will attract them to the electrostatic image. During contact between the electrostatic image and the developer mixture, the toner particles are stripped from the carrier particles to which they had formerly adhered (via triboelectric forces) by the relatively strong electrostatic forces associated with the latent image charge. In this manner, the toner particles are deposited on the electrostatic image to render it visible.
It is generally known to apply developer compositions of the above type to electrostatic images by means of a magnetic applicator that comprises a cylindrical sleeve of conductive, non-magnetic material having a magnetic core positioned within. The core usually comprises a plurality of parallel magnetic strips which are arranged around the core surface to present alternating north and south oriented magnetic fields. These fields project radially, through the sleeve, and serve to attract the developer composition to the sleeve outer surface to form what is commonly referred to in the art as a xe2x80x9cbrushxe2x80x9d or xe2x80x9cnap.xe2x80x9d Either or both the cylindrical sleeve and the magnetic core are rotated with respect to each other to cause the developer to advance from a supply sump to a position in which it contacts the electrostatic image to be developed. After development, the toner depleted carrier particles are returned to the sump for toner replenishment.
Conventional carrier particles for use with fixed magnetic cores are made of soft magnetic materials. However, soft magnetic carriers do not deliver toner to the electrostatic image in a manner such that the benefits of the foregoing multi-bit printheads and gray scale rendering of halftoned images can be fully realized. The conventional developer system has a rigid nap which essentially sweeps across the electrostatic image during development. As a result, images toned on such conventional development systems have a xe2x80x9cbrushedxe2x80x9d like surface, and as a result provide images with more defects, i.e., satellite particles, oversized dots, and so on. The result is an image with far less actual gray levels than can be theoretically realized. The resulting images in many instances have a xe2x80x9cgrainy,xe2x80x9d relatively high contrast appearance and therefore are not as pleasing to look at relative to the image being reproduced by such system.
As can be seen, it would be desirable to develop methods and apparatus capable of providing halftoned images which provide an actual number of gray levels which approaches the theoretical number of gray levels that can be provided by a digital printhead and gray scale rendering system. Such methods and apparatus could provide higher quality reproduced images which are more visually pleasing to a viewer.
The foregoing objects and advantages are realized by the present invention, which provides an apparatus and a method for the generation of halftoned images with reduced image artifacts and increased number of gray levels. The apparatus is a copier or a printer with an electrographic print engine for printing variable density halftone images. The engine includes a controller with a rendering algorithm that groups sets of adjacent pixels into sets of adjacent cells where each cell corresponds to a halftone dot of an image. The algorithm operates in conjunction with a gray scale printhead as described below. In sending data to the printhead, the controller parses a scanned image and set the exposure for each pixel in accordance with a growth and density program. As part of the growth program, the algorithm selectively grows halftone dots from zero size to a desired size equal to or less than a maximum size. It grows the dots by increasing exposure of one pixel in the cell until the pixel reaches a first level of exposure. It repeats this step for the rest of the pixels until the cell is at its desired size and at an initial density. A fully grown cell has a certain density. If a higher density is desired, the algorithm changes to a second series of steps to increase the overall density of the cell. It selectively adjusts the fully grown cells by sequentially increasing the level of exposure of each pixel in the cell. The selective adjustment is made on one pixel and that pixel is raised one level. Density is further increased by repeating the last step with each pixel until all the pixels are adjusting upwards one level. If further adjustment is needed, another round of pixel-by-pixel increases are made until the cell is at the desired level or at its maximum cumulative density level.
The algorithm controls the pulse width of a timing circuit that turns on the light emitting diodes (LEDs) that generate the latent pixels on the image member. The timing circuit is part of a gray scale writer that has an array of LEDs for discharging areas of a charged image member. The latent image is carried past a developer station where the image is developed. The developer station includes a container which preferably holds a two-component developer including hard magnetic carrier particles and toner particles. A cylindrical magnetic roller is covered with a concentric sleeve. The roller and sleeve usually turn in opposite directions with the sleeve moving concurrently with the image. The sleeve picks up developer particles as it passes through a developer supply and gently applies the toner to the latent image on the image member. After development, the image is transferred to a copy sheet and fixed to the copy sheet at a fusing station.
The method of the invention includes a number of steps. The image is captured in a raster format that includes a plurality of pixels. The pixels are grouped into sets that form cells where each cell includes multiple pixels. The rendering algorithm manipulates the digital data to generate halftone dots of variable sizes and variable densities. The rendered dots are used to expose a photoconductive surface of an image member and create a halftone electrostatic latent image of the original image. That latent image contacts a rotating magnetic brush at a development station where the developer includes a hard magnetic carrier and a toner.
In another aspect, the invention concerns an apparatus for the electrographic generation of halftoned images. The apparatus comprises a multi-bit printhead, means for generating a halftone image of varying dot sizes and densities, and a rotating magnetic brush development system comprising a developer composition comprised of a hard magnetic carrier and a toner.